Heteroleptic complexes via solubility control: examples of Cu(ii), Co(ii), Ni(ii) and Mn(ii) complexes based on the derivatives of terpyridine and hydroxyquinoline.

We describe the construction of synthetically challenging heteroleptic complexes by capitalizing on the solubility properties of their corresponding favored homoleptic complexes. We demonstrate that the formation of a heteroleptic Cu complex based on 2,2':6',2''-terpyridine (Terpy) and 8-hydroxyquinoline (HQ) is not possible due to the insolubility of (HQ)Cu. Replacing HQ with 8-hydroxy-2-quinolinecarbonitrile (HQCN) enabled the solubility of (HQCN)Cu in acetonitrile, leading to the formation of the heteroleptic complex Terpy(HQCN)Cu, TQCu. Applying these conditions to the synthesis of the corresponding heteroleptic Co complex resulted in TerpyCo(acetate), which is insoluble in acetonitrile. Upon changing the solvent to methanol, the carbonitrile group of HQCN was converted to carboxyimidate HQOMe yielding a heteroleptic complex Terpy(HQOMe)Co, TQ'Co. Using this method, we also generated the heteroleptic complex TQ'Ni and the polynuclear heteroleptic complex Q'4Q''2Mn4 (Q'' = HQOMe). Detailed analysis of the complexes included characterization by X-ray diffraction, EPR, UV-Vis, high resolution ESI MS, DFT calculations and electrochemistry. X-ray analysis of TQCu revealed distorted square pyramidal geometry, while TQ'Co and TQ'Ni exhibit distorted octahedral geometry, which includes metal coordination via the carboxyimidate nitrogen site. Interestingly, Q'4Q''2Mn4 was found to contain a [Mn(μ-O)(μ-O)N] core, which adopts a distorted octahedral geometry, and two types of HQ chelators. Thus, Q'4Q''2Mn4 is also heteroleptic even though it does not contain a Terpy ligand. Solution studies revealed that while TQCu is stable in solution, TQ'Co and TQ'Ni go through ligand exchange and are partially converted to their corresponding homoleptic complexes. Based on these data we could propose a mechanism for the formation of TQ'Co and TQ'Ni and show that TQ'Co can be prepared directly from Terpy and HQOMe.